Why do multicellular organisms need specialized systems for the intake of energy and exchange of materials, unlike unicellular organisms?
Most internal cells lack direct contact with the external environment
A unicellular organism such as Amoeba or Paramoecium is a single cell that lies directly in its surroundings. Oxygen, food and waste can pass in and out through the cell surface by simple diffusion, so no separate transport organ is needed.
In a multicellular body most cells lie buried deep inside the organism. These interior cells are not in touch with air, water or food. Diffusion alone cannot supply them fast enough over such large distances.
To bring materials to every cell, multicellular organisms evolve specialized systems: a digestive system for food, a respiratory system for gas exchange, a circulatory system for transport and an excretory system for waste removal.
The other options are factually wrong: single cells do require energy, body weight is not the criterion, and many multicellular organisms move freely.
Hence, multicellular organisms need specialized systems because most internal cells lack direct contact with the external environment.
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